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<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        function queryData(url) {
            return new Promise(function (resolve, reject) {
                var xhr = new XMLHttpRequest();
                xhr.onreadystatechange = function () {
                    if (xhr.readyState != 4) return;
                    if (xhr.readyState == 4 && xhr.status == 200) {
                        resolve(xhr.responseText)
                    } else {
                        reject('出错了');
                    }
                }
                xhr.open('get', url)
                xhr.send(null)
            });
        }
        // queryData('http://localhost:3000/data')
        //     .then(function (data) {
        //         console.log(data);
        //     }, function (info) {
        //         console.log(info);
        //     })
        //---------------------
        //发送多个ajax请求  并保证顺序
        // queryData('http://localhost:3000/data')
        //     .then(function (data) {
        //         console.log(data);
        //         return queryData('http://localhost:3000/data1')
        //     })
        //     .then(function (data) {
        //         console.log(data);
        //         return queryData('http://localhost:3000/data2')
        //     })
        //     .then(function (data) {
        //         console.log(data);
        //     })
        //---------------------------------
        queryData('http://localhost:3000/data')
            .then(function (data) {
                return queryData('http://localhost:3000/data1')
            })
            .then(function (data) {
                return new Promise(function (resolve, reject) {
                    setTimeout(function () {
                        resolve(123)
                    }, 1000)
                })
            })
            .then(function (data) {
                return 'hello'
            })
            .then(function (data) {
                console.log(data);
            })
    </script>
</body>

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